escapeshellcmd

(PHP 4, PHP 5, PHP 7)

escapeshellcmd — Escape shell metacharacters

Description

stringescapeshellcmd
( string$command
)

escapeshellcmd() escapes any characters in a
string that might be used to trick a shell command into executing
arbitrary commands. This function should be used to make sure
that any data coming from user input is escaped before this data
is passed to the exec() or
system() functions, or to the backtick
operator.

Following characters are preceded by a backslash:
&#;`|*?~<>^()[]{}$\, \x0A
and \xFF. ' and "
are escaped only if they are not paired. In Windows, all these characters
plus % and ! are replaced by a space instead.

escapeshellcmd() should be used on the whole
command string, and it still allows the attacker to pass
arbitrary number of arguments. For escaping a single argument
escapeshellarg() should be used instead.

Changelog

Version

Description

5.6.0

The default value for the encoding parameter was
changed to be the value of the
default_charset configuration
option.

User Contributed Notes 10 notes

There is a quirk to be aware of regarding use of echo. If you have a command which you want to execute which takes input from STDIN, you would normally do:

<?php $output = shell_exec("echo $input | /the/command"); ?>

Unfortunately, this is a *bad idea* and will make your script unportable, providing a very hard-to-trace bug on some systems. Depending on how the server is set up, /bin/sh will either call /bin/bash or /bin/dash, and these have very different versions of echo. Never use echo; use printf instead which is consistent. How do you escape for printf? Do this:

<?php
$input = 'string to be passed *exactly* to the command';
//Escape only what is needed to get by PHP's parser; we want
//the string data PHP is holding in its buffer to be passed
//exactly to stdin buffer of the command.
$cmd = str_replace(array('\\', '%'), array('\\\\', '%%'), $input);
$cmd = escapeshellarg($cmd);

$output = shell_exec("printf $cmd | /path/to/command");
?>

For the paranoid, this torture test verifies that both shell escaping and printf's own escaping are handled correctly. Use with confidence!

Mind it does not escape ! (exclamation mark). So if you want to i.e. printf() commands for later use in shell (i.e. by pasting to the console) you need to escape all exclamation marks or shell will try to process ! as history reference. This approach shall suffice:

"normal any user on linux can view almost any directory so:ls / -als will print a complete list of any file in the linux filesystem including its size, security and hidden files as well."ls / -alsR is the whole filesystem.

If you put your encoded filename into double-quotes as they suggest, then it will break on certain characters in filenames, such as ampersand.

For example if you have a filename called "foo & bar.jpg" and you use this function on it, your resulting filename when double-quoted will produce this and not be found:

"foo \& bar.jpg"

If you need to have a single argument where spaces are included then do not use this function with added double-quotes, use escapeshellarg() which encloses the whole string in single quotes.

I do not understand which purpose this particular function is intended for. I can't see any use for it, unless you pass it through another function and convert spaces " " to "\ ", which would allow you to use the string directly on the command line.

This function is great -- except when you need to legitimately use an escaped character as part of your command. The code below leaves the parts of the command that are enclosed within single quotes alone, but escapes the rest eg:

"echo Never use the '<blink>' tag ; cat /etc/passwd"becomes:"echo Never use the '<blink>' tag \; cat /etc/passwd"and not:"echo Never use the '\<blink\>' tag \; cat /etc/passwd"

i.e, we really want the ';' escaped, but not the HTML tag. I really needed the code below in order to run the external ImageMagick's 'convert' command properly and safely...

Well guys, i find it very hard that escapeshellarg and escapeshellcmd are forcely run when passing a command to exec, system or popen, when safe_mode is turned on.

Right now, i did not find any working solution to pass commands like this:cmd -arg1 -arg2 "<BLA varname=\"varvalue\" varname1=\"varvalue1\" />"

it is just the case, that the parameter for arg2 which is a string that looks like an HTML-Tag with various attributes set, all attributes of the string in arg2 gets splitted by the whitespaces within. this wont happen with safe_mode turned off, so it must be one of the escapefunctions, that breaks functionality.

In order to circumvent this, i have made a temporary solution, which dynamically creates a skriptfile (by fopen), which just contains the whole command with arguments, and then execute that skriptfile. i dont like that solution, but in the other hand, safe_mode cannot be easily turned off on that server.

the main reason for quoting a command is that it not multiple command can be joined. i don't know for sure if this is the right syntax but remeber that this can do some nice security breaks. here's one way of how to know exactly what your trying to break into for.

normal any user on linux can view almost any directory so:ls / -als will print a complete list of any file in the linux filesystem including its size, security and hidden files as well.

now the output would only become known to php and never will the user be able to view this data unless the php script would actual start to print it out. like passtru does!! but a good php coder knows never to use passtru unless not otherwise possible.

but what would happen if you can direct the output from ls also from that same commandline to a file in the webroot most webserver still default their base-webroot to /var/www/ so storing it there in text file to download it later and you can simply take coffee while checking wich files can be read by php security mode and then simply use the cp command to copy those to the webroot and download them to your own hard-disk. without a list of the files you can only guess where to copy from! and thats harder then guessing the root password.

so if the first command was quoted it is not possible to attach another command because of a syntax error. think of all the thinks you can do once you got a complete list of every file on the filesystem. including mounted once via NFS and others. security starts at keeping the door hidden.

also another nice command for hanging the webserver can be "php <?php while(true){ exec('ls / -als'); }; ?>" this keeps creating a file list on the entire filesystem wich not only keeps the hard-disk(s) bussy but also memory and cpu wich must store the returned list. so keeping in mind not all command accepted from users can be used blind.

actualy never accept any command from external sources only proven built-in predefined commands should be executed.